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Perl Development I ve been using Perl for many years. I ve found the language extremely useful in server and network administration. Without Perl, my job would ve been impossible to do. MySQL has an application-programming interface (API) for Perl. It s called the Perl Database Interface (DBI), and programmers can use it to create powerful applications that perform every function imaginable from administration tasks to reporting to Web applications with a MySQL server. The Perl-DBI combines full access to the power of MySQL with the ease of Perl. The DBI allows access to many different database types. Note To enable access to MySQL, however, a MySQL-specific database driver (DBD) must be used. This chapter refers to the combination of DBI-and-DBD as either the DBI or Perl-DBI. This chapter builds a few applications that take advantage of the Perl-DBI. Before building the applications, I show you the Perl API itself and the many functions included with it. The example applications stick to the fundamentals of using the Perl DBI and the MySQL DBD. For the time being, you can forego building a guestbook application (a common exercise in books devoted to Perl). If you want to build a guestbook later, the examples in this chapter provide a good foundation for doing so. 15 1515 CHAPTER …. In This Chapter Installing the Perl DBI and the MySQL DBD Introducing the DBI Using functions with the DBI Building basic applications Building Web applications Producing a Web site for e-commerce ….
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PART Development IV IVIV …. In This Part Chapter 15 Perl Development Chapter 16 PHP Development Chapter 17 ODBC/JDBC ….
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390 Part III . Administration . Indexes can help speed up operations with databases. . The EXPLAIN statement is used to look at the indexes that a SELECTstatement can use or might benefit from. . You can create indexes during the table creation statement or through the use of a separate CREATE INDEX statement. . The OPTIMIZE TABLEstatement is used to repair and defragment MyISAMand BDB tables. . MySQL performs many optimizations in the background without any user intervention. Statements are analyzed before they are run to determine validity and how to phrase them better for improved performance. …
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Chapter 14 . Performance Tuning 389 Figure 14-15: Examples of the myisamchk command to sort indexes on tables within the ecommerce database Summary The use of various MySQL logfiles is a major asset when troubleshooting the server or dealing with other database problems. . The error log is an important tool for helping to determine why the server crashes or won t start. . The binary update log and the query log can help you watch for updates that cause problems; they can also help you look for any SELECT statements that create performance problems. . The slow query log shows queries that take longer than the specified time or don t use an index. . The mysqladmin command contains useful functions for troubleshooting the server including flush functions and status and variable functions. . The flush functions with mysqladmin reset or clear counters, logfiles, and even the MySQL privilege system. . The MySQL CLI also includes flush functions along with functions to show variables and monitor server performance.
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388 Part III . Administration Figure 14-14: The OPTIMIZE TABLE statement in action Optimizing Queries MySQL performs many optimizations for queries and statements without the user having to be involved. These include processes like examination of clauses and statements to determine how they could be phrased in a more efficient manner, and rejection of some statements without running them if they are invalid. MySQL also performs background optimization of DISTINCT statements and JOIN statements. If this process is automated, what s your role? In the previous section I examined the EXPLAINstatement. The EXPLAIN statement can help you determine the indexes to use with a SELECTstatement. Useful indexes are the key to improving server performance when your database applications perform many reads. After the database has been populated, sorting the indexes within the database may be helpful. You can do this with the myisamchk statement. For example, to sort on the first index, issue the following command: myisamchk –sort-index –sort-records=1 Figure 14-15 shows the previous command when run against a few tables within the ecommerce database. The second attempt in the example fails because no indexes exist in the cardinfotable.
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Chapter 14 . Performance Tuning 387 The Possible_keysand Keyscolumns in the output of the EXPLAINstatement show possible keys that MySQL could use for the SELECT and the actual key (if any) that MySQL used for the query. All index types in MySQL are stored in B-Tree format. They can improve performance with a WHEREclause. Since most SELECTstatements will use a WHERE clause, an index can be the single most important improvement you make for better performance. Indexes can also quickly sort, locate the maximum and minimum values for a column, and assist with a join of two or more tables. The syntax for creating an index during a CREATE TABLE statement is CREATE TABLE
(, …, index (column(s) to include in index) ); Note Creating a column as a PRIMARY KEY also enables MySQL to optimize queries. You can also create an index after the table has been created through the create index or alter table commands. The create index command is actually an alias for an alter table statement. The syntax for a create index statement is as follows: CREATE INDEX on
(column(s) to include); Save disk space by choosing the correct column types and making the data as small as possible. Choosing the smallest or most appropriate column type helps MySQL work efficiently with the data. Choosing fixed-length columns can also increase Fixed length columns may end up wasting space since MySQL will always allocate the defined amount of bytes for the column regardless of the actual data length. The OPTIMIZE TABLEstatement run with the MySQL CLI is used to defragment a table or tables. The statement only works with MyISAMand BDB tables. OPTIMIZE TABLE repairs any deleted or split rows and also sorts indexes. Two examples of the OPTIMIZE TABLE statement are shown in Figure 14-14. speed. Note Caution While the OPTMIZE TABLE statement is running, the table is locked. Be aware that this can take a fair amount of time, depending on the size of the table.
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386 Part III . Administration Optimizing Databases and Tables Certain optimizing techniques can change the structure of databases and tables to make them operate more efficiently. In addition, certain statements can optimize tables with performance enhancements such as defragmentation. The use of indexes will help performance immensely on large datasets. MySQL stores indexes and data in separate files to maximize efficiency. Adding an index (or creating one with a table) will help when performing operations on the data; that way MySQL need only look at a subset of the data for the correct result. If there is a PRIMARY KEY index on the table, it should be a short task to maximize speed. If you frequently search on multiple columns, index the columns with the most used column being the first in the list. However, don t make unnecessary indexes. The EXPLAIN statement can assist in determining when an index might be useful. The EXPLAIN statement is used with a SELECT statement. For example, Figure 14-13 uses the EXPLAIN statement to show indexes used by a SELECTstatement. Figure 14-13: A description of two tables in the ecommerce database along with an EXPLAIN statement to examine the SELECT statement being used to find the first and last names of customers
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Chapter 14 . Performance Tuning 385 Figure 14-11: Using the mysqlshow command to show the tables within a database Figure 14-12: Using the mysqlshow command to list the columns within a table of the ecommerce database
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384 Part III . Administration mysqlshow Though complemented by SHOWstatements in the CLI, the mysqlshow command can find the structure of databases, tables, and columns. The mysqlshow statement is unlike other commands in MySQL. You can t simply type the command to get a syntax printing. To see a syntax reference for the mysqlshowcommand, you should add the –help switch, as shown in Figure 14-10. Caution The -? switch is listed as a synonym for the –help switch, but it doesn t work. Figure 14-10: A syntax reference for the mysqlshow command is obtained by adding the –help switch. Using the mysqlshow command is quite similar to using other commands with MySQL: The -u switch defines the user, the -p switch can define the password, the -h switch defines the host, and so on. The output from the mysqlshow command is similar to the output from the equivalent CLI command (such as SHOW TABLES or SHOW COLUMNS). Examples of the output from the mysqlshow command appear in Figures 14-11 and 14-12.
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Chapter 14 . Performance Tuning 383 Figure 14-8: The FLUSH TABLES statement using specific tables The third variation of the FLUSH TABLESstatement is as follows: FLUSH TABLES WITH READ LOCK
The FLUSH TABLES WITH READ LOCKstatement closes all open or in-use tables, just like the FLUSH TABLES statement. However, the FLUSH TABLES WITH READ LOCK statement also places a lock on the tables that must be manually unlocked with the UNLOCK TABLESstatement. Figure 14-9 shows an example of the FLUSH TABLES WITH READ LOCK statement, an attempt to read from one of the locked tables, and then the UNLOCKTABLES statement to allow access again. Figure 14-9: An example of the FLUSH TABLES WITH READ LOCK statement.
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